著者

幾原 雄一 栗下 裕明 吉永 日出男

出版者

公益社団法人日本セラミックス協会

雑誌

窯業協會誌 (ISSN:18842127)

巻号頁・発行日

vol.95, no.1102, pp.638-645, 1987-06-01

被引用文献数

9 18

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In order to in vestigate the effect of grain boundary structure on the high-temperature strength of SiC, three kinds of SiC materials were prepared by pressureless sintering; material A with sintering aids of B+C, material B with B+C+AlN, and material C without any sintering aid. Their strength was measured by three-point bending at temperatures from room temperature to 2070K. The grain boundary structure was observed by HR-TEM. The following results were obtained.<br>(1) The strength of material A increases with increasing temperature up to 2070K. The fracture mode is transgranular at all temperatures studied. At the grain boundaries observed, a non-crystalline phase of 2-5nm in thickness is always found. This phase is thought to be either a compound of B and C or an extended grain boundary.<br>(2) The strength of material B increases up to 1770K, but above that temperature it decreases rapidly. The fracture mode also changes at that temperature from transgranular to intergranular. There exists also a 2-5nm thick non-crystalline phase at grain boundaries. The boundary phase is thought to be a compound in the system B-C-AlN. Above 1770K this phase is considered to flow viscously under stress to bring about boundary sliding which causes the strength to decrease.<br>(3) The strength of material C is almost independent of temperature. There exists again a 3-5nm thick non-crystalline phase at grain boundaries, but the boundary phase is thought to be an extended grain boundary.<br>(4) The dihedral angles observed in material C are frequently much larger than the critical angle of 60°. This observation is against the Prochazka's thermodynamic limitation, γ_gb/γ_sv<√3. The large dihedral angles may come from the existence of a grain boundary phase, which lowers the boundary energy.<br>(5) Densification of materials A and B is thought to proceed by the diffusion through the grain boundary phases.